When booting up my Acer Aspire 1690 laptop the fan starts but then quits after about 5 seconds. The system starts up nicely (OS starts up) it just doesn't have a fan anymore which basically renders the laptop useless.

I already tried to clean the fan with compressed air and I even ordered a new fan, but to no avail.

Anyone who had a similar problem? Or any idea how to fix it?

You seem to have done the right things so far. Now all that's left is to replace the fan once to receive the replaement one you ordered.
I have a thought about something you might try until you do the replacement, but no guarantees given: open up the laptop and prop it up on supports so that an external fan can blow on the CPU while you are working. Maybe it will provide enough cooling capacity to enable your PC to work. Just make sure that your laptop is stable and don't let anything touch the exposed innards and cause a short or other damage.
Cheers

you don't have to worry about that. It again starts when heat increased. So no need to replace CPU fan it's a default properties of laptop CPU cooler.

Thank you for the quick answers.

Maybe I should be a bit more specific about what happened and what I already did:
A few weeks ago when I wanted to start up my laptop (which I had for about 2 and a half year) it made a strange noise. It was my CPU fan which stopped working after 5 seconds while making somewhat of a strange noise (maybe it touches something).
I didn't trust this so I shut down the laptop and the next morning go to the local pc repair shop to have a look at it.
They took out the fan and cleaned it with compressed air but this didn't have any effect. I asked them to have it fixed (by replacing the fan) but apparently they can't do this and I have to send it to Acer.

A few days later (after setting up a backup PC + backing up my files) I file a repair case at Acer. But because they can't give me any indication about how much it was going to cost me + I don't like to send my laptop by mail I try to fix it myself.

I ordered a compatible fan and received it by mail a week later. Replaced the fan (only included the fan so had to connect the heatsink myself) and started up the laptop.

Same story... after 5 seconds the fan shuts down but it didn't make that strange sound anymore. So I thought maybe this was normal (it might start up again when the CPU gets too hot) and tried to use the laptop. It even seemed to have a light breeze coming out of the fan (but I couldn't hear it running).

But after about 10 minutes the laptop shut down without any warning (Ubuntu) so I suppose the CPU got too hot and the fan isn't working.

Tonight I'll try to use a program to monitor the heat of the CPU to see what happens. Maybe then I can be sure the fan isn't working.

But I like the idea of using an external fan; how should I set this up? How can I power an external fan? Or maybe I can just use the current fan but power it by an external source?

From reading through a different thread, I learned that sometimes the heat transfer cream between the CPU and the heat-sink dries out. When this happens, even if the fan is working, the heat will not be efficiently dissipated. In the other thread the recommendation (which solved the problem) was to buy and re-apply heat transfer cream to the heat-sink-CPU contact area and close up the PC.
How about trying that?

Yes, I also read about that but the problem is that the fan doesn't even go to full speed (if it is running at all).
So I don't think adding transfer cream would have much use.

Any other suggestions?

Can you go into BIOS and see if the CPU is over heating?

On my system I can adjust fan speed and what the CPU target temperature is (maybe try lowering it).

Are you using any other OS, I know vista and win7 you can lower the CPU utilization thus lowering the temperature emitted. -There maybe something like this in Ubuntu but I have never need to lower the CPU utilization for that.

It may not solve the problem, but it may make your computer more usable.

BTW thermal paste should help dissipate the heat even if the fan isn't working correctly or fast enough.

This morning I tried to keep the laptop on again (with the new fan) and this time I made sure I could see the fan (laptop was open). And all of a sudden it works just fine :-)

I kept it running in Windows XP though because I can monitor the CPU heat there; maybe tonight I will try to keep it running in Ubuntu.

Thanks for all the help.

btw if I mark this item as solved can I put it on unsolved again when the same problem pops up again?

This sounds like a loose connection problem. Check all your connections carefully and continue running the computer open for a while (until you feel confident that the problem has "worked itself out"). Then close it up and (hopefully) happy computing.
You can always start a new thread and start it by giving the background story to bring people up to date.

Typing this from my laptop now, in Ubuntu.
The fan seems to be spinning nicely; the laptop case is still open though.

I'm thinking about buying some thermal paste just to make sure.

Thanks for all the help.